The present invention relates to a signal processing device and to a hands-free call device including the signal processing device.
In a hands-free call device provided with a function for carrying out voice-amplified calls using a speaker and a microphone instead of a handset, communication may be perturbed when the speaker and the microphone are used simultaneously, as a result of which there forms a closed loop through acoustic coupling such that howling occurs when the gain of the closed loop exceeds 1. In order to ensure stable communication in such a hands-free call device provided with a voice-amplified call function, therefore, some scheme must be resorted to for constantly keeping the closed loop gain at or below 1.
A voice switch is one such known scheme. In a voice switch, the closed loop gain is kept at or below 1 by monitoring the transmission voice signal level and the reception voice signal level and by increasing the signal attenuation of the smaller signal level. Voice switches, in which computations are simpler than in echo suppressors, are advantageous in that they can be built easily, since they not require a large memory capacity for storing adaptive filter coefficients, status variables and the like. Documents describing voice switches include, for instance, Japanese Unexamined Patent Application Laid-open No. H10-93681.
In a conventional voice switch, however, the mode that is to be operated is discriminated between a voice transmission mode and a voice reception mode, by comparing the transmission voice signal level and reception voice signal level, and hence the echo of the reception voice outputted by the speaker enters the microphone and becomes superposed with the transmission voice, which makes it difficult to tell apart the echo of reception voice from the transmission voice. This means that it is difficult to judge the call direction in environments where echo is likely to occur. When a switch is switched, upon mistaking of the above echo with audio from a near-end speaker included in the transmission voice, the audio of the far-end speaker included in the reception voice is interrupted while being generated, which impairs the quality of the call.
The circuit constitution of a conventional voice switch, moreover, is extremely complex, and hence installing a voice switch in a handset was difficult. In some hands-free call devices, signal processing relating to the hands-free operation is carried out with a handset and a base set working together via wireless communication. Such devices are problematic in that the hands-free call can be perturbed should wireless communication fail.